Which of these numbers is prime? ${7,\ 38,\ 45,\ 55,\ 65}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 7 are 1 and 7. The factors of 38 are 1, 2, 19, and 38. The factors of 45 are 1, 3, 5, 9, 15, and 45. The factors of 55 are 1, 5, 11, and 55. The factors of 65 are 1, 5, 13, and 65. Thus, 7 is a prime number.